The present invention relates generally to apparatus for determining oxygen saturation levels in blood, or oximeters, and more particularly to apparatus suitable for use with in vivo catheter-type oximeters which determines the oxygen saturation level by utilizing reflected electromagnetic radiation at a plurality of different wavelengths.
Various catheter-type oximeter apparatus are known for determining blood oxygen saturation. One such apparatus is disclosed in U.S. Pat. No. 4,144,604 to Shaw, et al., the disclosure of which is incorporated herein by reference. Typically with such apparatus, a catheter is introduced into a blood vessel and the blood within the vessel flows about the catheter tip. The catheter includes a first fiberoptic guide which conducts radiation from the oximeter apparatus to an aperture at the catheter tip. The blood flowing about the catheter tip scatters a portion of the incident radiation thereon back to a second aperture at the catheter tip where a second fiberoptic guide transmits this back-scattered radiation to the oximeter apparatus. The back-scattered radiation is then analyzed by the oximeter apparatus to provide a measurement of oxygen saturation.
As the blood under test flows about the catheter tip, the amount of radiation returning therefrom exhibits pulsatile fluctuations synchronized with the heartbeat. It is believed that these fluctuations result from the catheter tip impacting or very closely approaching the vessel wall. Since the vessel wall exhibits reflective characteristics not necessarily related to blood oxygen saturation, these substantial fluctuation resulting in greatest part from the reflectance of the vessel wall can introduce inaccuracies into the measurement of blood oxygen.
Various techniques have been utilized in oximeters in the past which have resulted in reducing the effects of or eliminating these substantial fluctuations. In one system, the time constant or span of the oxygen saturation measurement is relatively long thereby reducing the influence that the substantial radiation fluctuations may have on the oxygen saturation calculation. However, this technique does not remove from the oxygen saturation measurement process the erroneous data represented by the substantial radiation fluctuations. Thus, the oxygen saturation calculation is based at least in part upon erroneous data, thereby adversely affecting the accuracy of the oxygen saturation measurement.
In another oximeter apparatus, the catheter tip is enclosed within a cage-type structure in an attempt to prevent the oximeter tip from impacting the vessel wall. While the cage structure may effectively separate the catheter tip from the vessel wall, the structure also greatly increases the tendency for deposits to form on the catheter tip, an undesirable result.